1. The Field of the Invention
The present invention relates to a method and a circuit arrangement for adjusting an active filter in determining frequency response.
2. The Relevant Technology
Active filters are used, for example, in telecommunication networks. In this case, active filters are used, particularly for digital transmission methods, as input and output filters upstream of an analogue-digital converter and downstream of a digital-analogue converter, respectively. In a digital technology application, it is important for a particular nominal frequency response to be observed on the basis of the clock frequencies used in the digital system. If the elements used in an active filter to determine frequency response are capacitors, inter alia, which disadvantageously vary greatly in production, it is generally necessary to have a means of alignment for setting the frequency response. Normally, such active filters for use in telecommunication networks are integrated in a semiconductor together with an alignment circuit.
By way of example, VDSL systems require an active low-pass filter whose cut-off frequency needs to be able to be set to one of a plurality of frequencies between 8 and 12.44 MHz, the permissible tolerance being ±5% of the frequency value. If a set of capacitors which determine frequency response is now used for each adjustable nominal frequency together with the respective alignment circuit required, the result is disadvantageously a very high level of circuit complexity, whose scope entails further drawbacks, such as parasitic capacitances.